98 will have no effect at all on the PCV system or the valves as no fuel passes over them. Fuel octane rating relates to its resistance to knock (pre-ignition), not the potential energy within the fuel and how far you will be able to go on a gallon
Too many variables over a tank of fuel to draw a conclusion from 2 tanks worth anyway. You won't notice much difference on a standard car (few hp) but it's a lot more important once mapped and making use of more timing advance